An undesirable feature of many hot melt glue guns is that they leak from the nozzle opening on warm-up as well as during normal use of the gun. In the typical construction of a glue gun, a user feeds a glue stick at room temperature into a relatively short melt chamber having a temperature in the range of 250° F. to 400° F., depending on the model and the glue stick formulation.
A fundamental property of thermoplastics is its volumetric expansion as a function of temperature—commonly called thermal expansion. The coefficient of thermal expansion for most thermoplastics is known. For most EVA based formulas, the rate of thermal expansion is in the range of 100 micro-inches/inch/degree F., so this translates to about a 5% volumetric expansion.
As the user feeds a glue stick into the melt chamber, the temperature of the melt chamber temporarily drops, because the melt chamber must heat the glue stick from a temperature of about 75° F. to, for example, 350° F. in a relatively short period of time. As the temperature of the glue increases it expands, and the more quickly glue is fed into the melt chamber, the more it is affected by this thermal expansion. This volumetric expansion is the primary cause of leaking in the glue guns.
Use of a glue gun lightly requires only a small amount of cold stick be heated, resulting in a relatively small amount of thermal expansion in a given time period. However, when a glue gun is used heavily, such as when multiple glue sticks are fed serially into the melt chamber rapidly, each complete stick must be heated in a short period of time, resulting in thermal expansion of a large amount of glue in a short time.
For a typical glue stick having a diameter of about 0.450″, the thermal expansion can be calculated to be theoretically about 0.15″ of linear expansion, which is about 0.025 cubic inches of glue available to drip or drool. For a bead of glue 0.125″ in width, a bead of about 2″ in length can form from the glue that is available to drool just from thermal expansion. In practice, the glue stick naturally retracts a little bit when dispensing pressure is released, resulting from the release of the pressure on the glue stick that must be applied to open the ball check-valve in the nozzle during dispensing. This slight retraction reduces the pressure in the melt chamber, and the length of a bead of glue after the user stops feeding the glue stick into the melt chamber is in practice typically about 1 inch as the pressure relief is satisfied and the dispensing stops gradually.
In addition many glue guns experience thermal expansion of the solid glue that is already within the chamber on warm up, resulting in an inevitable drip.